The effectiveness of birth control pills depends entirely on how they are used. While it is usually claimed that effectiveness is in the range of 96 to 99%, the reality is that effectiveness is around 87%. The reason can be traced to poor user habit. This discrepancy in effectiveness could be taking the pill at different times in the day, missing days or skipping the pill. There are other reasons why the effective of the pill could well be reduced. This is the pill’s incompatibility with certain other medications.
The point to be noted is that the pill has to be taken at the same time every day. For example if the woman forgets to take just one pill then the chances of becoming pregnant increases. During the cycle if two or more pills are missed then there is further chance of becoming pregnant.
There are two kinds of contraceptive pills and their efficacy is naturally different. One of the commonest is the combined oral contraceptive pill which contains both oestrogen and progesterone. If all instructions are followed then the failure rate is about 0.2 per cent per year. Then there is the Progestogen –only pill or POP ( it is also known as a mini pill) and this has a higher failure rate that can be anywhere between two and eight per cent per year.
Traditional birth control pills make the woman’s reproductive system mimic a regular 28-day monthly cycle. For the first 21 days, the woman takes active pills containing reproductive hormones. For the last seven days, she takes a placebo. While she is taking the placebo pills, she will bleed vaginally, as if she were having a regular menstrual period.
By contrast, extended-cycle birth control pills contain active hormones for every day of the month. The newest extended-cycle regimen, Lybrel, is a low-dose pill that's designed to be taken continuously for one year, with no breaks for hormone-free intervals. Lybrel is meant to suppress all menstrual bleeding.
There is a pill specifically designed to keep the woman from becoming pregnant if she has had unprotected vaginal intercourse. An emergency contraception pill (Plan B) — also called the "morning-after pill" — contains a higher dose of progestin than is found in other birth control pills and is available over-the-counter for women age 17 and older.
There are other reasons why the pill is not effective and these include: (a) nausea or vomiting when the pill has not been properly absorbed into the system, (b) interaction with other drugs. For example anti-epileptic drugs like phenobaritone and phenstoin and fungal infection drugs like carbamazapine and griseofulvin and other broad spectrum antibiotics will all have an adverse effect on the effectiveness of the combined contraceptive pill.
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